1. Field of the Invention
This invention relates generally to apparatus for cooling gas and more particularly to apparatus f or cooling the EGR gas by means of engine coolant, car air conditioner refrigerant, and cooling air.
2. Description of the Prior Art
A method of collecting part of exhaust gas from an exhaust system coupled with a return thereof to an engine's air intake system for addition to a fuel-air mixture is known as the EGR (Exhaust Gas Recirculation). Inasmuch as the EGR is highly conducive to inhibiting generation of nitrogen oxides, reducing pump loss, diminishing heat discharge to the coolant concomitant to a temperature drop of the combustion gas, increasing the specific heat ratio due to changes in the volume and composition of operating gas, and improving the resultant cycle efficiency, this is considered to be an effective method of improving the thermal efficiency of engines.
However, it is also known that a rise in the temperature of the EGR gas may cause degradation of the durability of the EGR valves and other related parts through the heat action thereof to produce breakage at an early stage. To remedy such situation, devices for cooling the EGR gas by means of engine coolant or cooling air have been proposed, and multi-tube heat exchangers are currently commercially available.
An example of the multi-tube heat exchangers currently employed in this instance is shown in FIG. 7. Namely, on one of the left and right ends or both ends of a heat exchanger body, there is provided a cap having an inlet of gas or an outlet thereof, which is partitioned via a divider wall. A barrel is fixedly connected to head members (hubs) containing a separate inlet of a cooling medium, and inside the barrel are a multiplicity of heat transmission tubes fixedly placed in proper order at connecting holes which are provided on the divider wall on both ends mentioned above. A nipple screwed to the inlet of the cooling medium and an outlet thereof is connected to a branch pipe such as a rubber hose, through which an engine coolant or cooling air is introduced or discharged, thereby cooling the EGR gas flowing inside the heat transmission tubes (see Japanese Utility Model Gazette No. 309/82).
Nevertheless, this prior art structure of the above-mentioned multi-tube heat exchangers has a problem of a large flow resistance created as flow of the engine coolant or cooling air is sharply curved at the inlet of the cooling medium. Further, cast or forged construction of the head members and the divider wall to which the multiplicity of heat transmission tubes are secured contributes to making weight of the heat exchanger body excessively heavy. Moreover, additional work is required to produce connecting holes to screw the branch pipes to the inlet of the cooling medium and the outlet thereof, while still more steps are necessary to fixedly arrange in proper order the multiplicity of heat transmission tubes on the divider wall. Hence, it takes a great number of assembly steps with a consequent deterioration of workability. Another adverse factor of the conventional structure stems from brazing which is performed to connect the multiplicity of heat transmission tubes to the divider wall: a difference in the wall thickness between the heat transmission tubes and the divider wall indicates a difference in heat capacity, a cause of poor reliability for maintaining strength of the brazed sections, leading to chances of causing faulty brazing.
The applicant of the present invention proposed a novel apparatus for resolution of the above-mentioned problems in Japanese Patent Laid-Open No.267691/95 (filed on Sep. 21, 1995). As shown in FIG. 8, this is a multi-tube apparatus for cooling the EGR gas having a tube sheet secured to the inner wall of both ends of the barrel on which a plurality of heat transmission tubes are fixedly set up in proper order, and at the caps on the ends of the above-mentioned barrel there are provided an inlet of EGR gas and an outlet thereof. Furthermore, construction of the apparatus includes an inlet of the cooling medium and an outlet thereof on the barrel proper by means of burring towards the outside, while a plurality of branch pipes are joined to the inlet of the cooling medium and the outlet thereof through direct brazing or welding.
The EGR gas cooling apparatus proposed in Japanese Patent Laid-Open No. 267691/95 was effective in ameliorating the above-mentioned difficulties. This fact notwithstanding, since the EGR gas cooling apparatus was subject to vibrating environments due to engine, vibration generating during running as well as pulsation concomitant to pressure fluctuation of the EGR gas proper, stress tended to converge upon the joints between the heat transmission tubes and the tube sheet. It was also necessary to pay more consideration to the strength of the heat transmission tubes proper with respect to the above-mentioned vibration.
Many existing multi-tube heat exchangers designed for heat exchange between liquids are of a construction that disposes baffle plates at a plurality of locations in a longitudinal direction of the inner wall of the barrel, the baffle plates having through holes into which heat transmission tubes are inserted. In this case, flow of the cooling medium running outside the heat transmission tubes is made to take a detour via the baffle plates to enhance the heat exchange efficiency with the medium running inside the heat transmission tubes, thus necessitating certain sealing requirements, if not rigorous, between the heat transmission tubes and the through holes through which the tubes extend.
Be that as it may, the EGR gas cooling apparatus is essentially a device for cooling the EGR gas flowing inside the heat transmission tubes by exchanging heat thereof with the coolant or cooling air running outside the heat transmission tubes. Unlike a normal heat exchange exchanging heat between one liquid with another, especially when a coolant is used, the heat transmission coefficient outside the tubes (Kcal/m.sup.2 hr.degree.C.) becomes about 100 times that inside the tubes, that is, effects of the contact direction and contact time of a liquid in contact with the outer surface of the heat transmission tubes upon the cooling effect of gaseous matter circulating inside the heat transmission tubes are extremely low. Consequently, it was confirmed by experiments conducted by the inventor of this invention that it was hardly necessary to set up the baffle plates for making flow of the external fluid to take a detour so as to move the fluid in a direction perpendicular to the axes of the heat transmission tubes and to take into consideration the seal property between the heat transmission tubes and the through holes of the baffle plates.
When the baffle plates were placed in the EGR gas cooling apparatus in the same way as the conventional multi-tube heat exchangers for purposes of heat exchange between liquids, there was a possibility that construction of the apparatus with the plain through holes provided by drilling the baffle plates as shown in FIG. 10 or with the through holes formed by burring into which the heat transmission tubes were inserted as shown in FIG. 11, in the event of being subjected to the above-mentioned vibrating environments, might suffer more than necessary shock, thus leading to deterioration of the service life of the heat transmission tubes.